1. Technical Field
The present invention relates to a main rotor assembly for rotary wing aircraft. More specifically, the invention relates to segmented rotor blades wherein the collective pitch of the movable segments may be independently varied. Control inputs may be manual, as by the pilot, or may be made by a computer.
The beneficial force that overcomes or counterbalances the weight of an airplane and allows the craft to become and remain airborne is called lift. In helicopters, lift is provided by the rapid rotation of the main rotor blade configuration. The rotor blades are airfoils, meaning that they are designed to produce a resultant force of lift or thrust when air passes over them. Airfoils used in most aircraft are engineered to provide certain flight performance characteristics specified by the designers of the craft. Airfoil design may profoundly affect such performance characteristics as air speeds, maneuverability, lift, airframe vibration, and takeoff/landing capabilities. Often the components of the different flight performance characteristics clash with one another. Thus, one characteristic may be emphasized--steep takeoff and landing capabilities, for example--while another may have to be rectified in unison with the adjustments made to the first. Generally speaking, airfoil engineering and aircraft performance design require compromises. One of the most critical tasks in the design and engineering of an airplane such as a helicopter is that of judiciously compromising design characteristics in such a manner that the aircraft will have optimal capabilities in those performance areas deemed essential to the accomplishment of its mission.
A helicopter is also subject to an array of complex forces which may be detrimental to flight performance on the one hand and potentially destructive of the aircraft on the other. The main rotor assembly is a dynamic complex that receives a variety of severe stresses, including harmonic motion from vibrational resonance, lead-lag oscillations and flapping, turbulence from airflow patterns and Coriolis forces, resultant lift, and induced drag. Therefore, it is vital for the flight performance and also the durability and airworthiness of the craft that the upper and lower surfaces of the rotor be cambered in a manner precisely calculated to serve critical and conflicting aspects of the machine's complex aerodynamics.
The helicopter's main rotor provides lift and thrust, the forces needed for vertical ascent and horizontal movement. An aviator may change the magnitude of lift by adjusting the collective pitch of the helicopter's main rotor blades. Collective pitch may be decreased to a neutral acute angle at which the helicopter can hover. The stresses referred to above are present in all flight operations; they may be brought about by, or changed by, alterations in altitude, velocity, attitude and direction of the aircraft or alterations in climatic and atmospheric conditions. Thus, a change in the collective pitch of the main rotor blades will necessarily alter the forces and stresses to which the airfoils are subject. It must be emphasized in this context, moreover, that the forces and stresses on the airfoil are not uniform on the length of the span. The vortex of air at the rotor tip and the circulation of turbulent air along the span contribute to the nonuniformity. To illustrate this principle further, we can generally say that lift patterns vary between hovering, normal cruise, and critical airspeed operations (one quarter of rotor blade lagging edge approaches stall; stall can spread inboard if collective pitch angle is sufficiently great). It would be beneficial to trim the airfoils in such a way that a maximum efficiency could be obtained by localizing pitch variations to deal with the differing conditions at particular stations on the span. As presently constituted, however, the pitch angle of the entire span must be moved as an integral unit and much efficiency of flight performance is lost. It is this problem and the resultant loss in efficiency of flight that are addressed by the present invention.
2. Description of Related Art
Segmented airfoils have been developed and used on certain kinds of vessels or vehicles to deal with specific challenges raised by the use of rotating twisted radial blades in fluid media to provide forces capable of providing some kind of propulsive force. For example, U.S. Pat. No. 2,969,211 discloses collapsible, inflatable, wind-rotating accordion vanes of airfoil configuration to be used for braking the descent of a rocket-type vehicle on its reentry into the earth's atmosphere from the upper regions of the stratosphere. A configuration of blades applicable to air or water craft is disclosed in U.S. Pat. No. 2,065,254. This configuration provides for spiral rotary wing sections in staggered segments such that adjacent segments or sections may be set at different angles, as measured between the chord of the airfoil and the direction of the prevailing wind.
The use of airfoil configurations to dampen the various stresses to which a primary airfoil (such as the wings of an ordinary airplane) are subject is also known. U.S. Pat. No. 2,124,098, for example, discloses a pivotable airfoil suspended from the fixed wing of an airplane for damping or counteracting flutter forces affecting the wing. U.S. Pat. No. 2,332,516 teaches how flutter and vibration may be damped by setting the vibrational resonance period of a control surface, such as a flap, out of phase with the vibrational frequency of the wing and transmitting the harmonic motion through the control flap linkage.
In the related art, it is believed that fixed segmented rotor blades have been used on experimental helicopters.